1. Field of the Invention
This invention relates to a hydrostatic drive system with a demand-responsive pump and at least one hydraulic circuit or consumer connected to it. Each consumer can be actuated by a directional control valve which acts as a throttle in intermediate positions. The system includes a pressure-maintaining valve for the distribution of the delivery flow to the directional control valve independently of the load. The pressure-maintaining valve can be controlled by a differential signal which is formed from a load pressure signal and a delivery pressure signal.
2. Background Information
In drive systems to which the present invention relates, the volume flow which flows to the consumer (or hydraulic circuit) is determined independently of its load pressure by the opening width of the directional control valve. The volume flow which flows to the consumer is thereby proportional to the speed of movement of the consumer. To achieve a distribution of the delivery flow which is independent of the load when a plurality of consumers are connected, pressure-maintaining valves are located upstream of the directional control valves. The speed of movement of the consumer can therefore be controlled or regulated. In such drive systems of the prior art, there is no control of the drive force for consumers which work in translation movements or of the drive torque for consumers which work in rotational movements.
Prior art document DE-OS 36 43 110 discloses that the pressure of the hydraulic medium which flows to the consumer can be limited as a function of the displacement distance of the directional control valve, which does not have a pressure-maintaining valve to divide the delivery flow independently of the load. The prior art document teaches that a fixed throttle and a throttle which is variable as a function of the displacement of the directional control valve can be located in the control pressure signal line. When such a directional control valve is present, the pressure medium flows to a consumer at a pressure which is a function of the displacement of the directional control valve. The torque of a consumer with a rotating output shaft can thereby be specified by the displacement of the directional control valve. In this case, however, the load pressure signal is derived from the delivery flow of the pump, and is connected to a tank by means of the variable throttle. To generate the torque control in the directional control valve, a portion of the delivery flow is thereby converted into heat and into lost power. During simultaneous operation of an additional consumer which has a higher load pressure, the torque control mechanism is deactivated. The higher delivery flow which occurs at the pump on account of the higher load pressure signal, and thus also the higher delivery pressure, is reduced by the pressure relief valves which correspond to the consumer. The torque limitation control is thereby not operational under all operating conditions, and also entails losses in the pump delivery flow.
The object of the invention is to make available a hydrostatic drive system of the type described above which avoids the previous limitation of the force and/or torque of the consumer.